Ping, any thoughts on this.... On Wed, Aug 26, 2015 at 01:03:07PM +0100, Daniel P. Berrange wrote: > There are many problems in QEMU related to introspection > of features, which Markus has been attacking/slaying for > a while. One of the remaining elephants in the corner of > the room which I've not seen work on is QOM. > > QOM has a nice class/object framework, kind of like GLib's > GObject, but with the difference that properties are > registered against objects rather than classes. IIRC the > rationale for this is that in some cases the properties > registered are going to be dynamically decided at runtime, > so it isn't possible to statically define them against > classes. Now this may well be true of some properties, > but there are equally plenty of cases where the properties > /are/ invariant and could be registered against classes. > > There are two core problems with registering properties > against object instances: > > - It is wasteful of memory to duplicate the allocation > of ObjectProperty structs against each object > instance. When you have N object instances, you > have O(N) memory usage, instead of O(1). This is not > a problem for objects which are mostly singletons, > but there are cases in QEMU where you instantiate > many instances of the same class and/or have many > properties. > > - It prevents static introspection. Since the property > is only registered in the object initializer, there > is no way to programmatically query what properties > an object supports without first instantiating it. > Taking machine types as an example, if you want to > introspect every machine type supported by a QEMU > binary you thus have to launch QEMU many times, > passing a different -M argument each time. As the > number of different machine types & objects > increases this quickly becomes impractical. > > This series thus extends QOM to make it possible to register > properties against the classes, in addition to against object > instances. When looking up a property, a search will be done > starting at the base class, then each subclass in turn, and > finally against the object. Names are enforced to be unique > across the parent classes for sanity. > > This only currently supports simple scalar properties where > the actual property value storage is managed by the object > instance. The object child and object link properties use > implicit storage in the ObjectProperty struct's 'opaque' > field, so we can't allow those against the class. Solving > this is doable, but more work, so is left as an exercise > for the future. > > The first patch adds the neccessary QOM functionality. The > following 6 patches then illustrate the fairly trivial > conversions of a bunch of objects. > > The eventual goal ought to be that everything is registered > against the class, leaving only the (hopefully few) cases > where per-instance properties are truely needed unconverted. > > This series doesn't attempt to implement introspection > either - this would require a new QMP command such as > 'qom-list-type-props' to query the properties against > classes. > > I'm not really planning to spend much more time on this > myself. I'm just using this series to illustrate how I > believe the introspection problem in QOM can be fairly > easily addressed. Hopefully this will stimulate some > discussion & interest in doing the full job.... > > Daniel P. Berrange (7): > qom: allow properties to be registered against classes > hostmem: register properties against the class instead of object > rng: register properties against the class instead of object > tpm: register properties against the class instead of object > cpu: avoid using object instance state in property getter > x86-cpu: register properties against the class instead of object > machine: register properties against the class instead of object > > backends/hostmem-file.c | 26 +++--- > backends/hostmem.c | 41 ++++----- > backends/rng-egd.c | 12 +-- > backends/rng-random.c | 10 +-- > backends/rng.c | 14 ++- > backends/tpm.c | 12 +-- > hw/core/machine.c | 193 +++++++++++++++++++-------------------- > include/qom/object.h | 44 +++++++++ > qom/object.c | 233 > ++++++++++++++++++++++++++++++++++++++++++++++-- > target-i386/cpu.c | 88 +++++++++++------- > 10 files changed, 476 insertions(+), 197 deletions(-) > > -- > 2.4.3 >
Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|
